One example of a component where static pressure measurement is required is a bearing housing located at the front end of certain gas turbine engines. The bearing housing has an inner ring 100 of a strutted case assembly, as illustrated in FIG. 1. The inner ring houses the bearing and includes a plurality of radial struts connecting it to an outer ring which in turn connects the fan case assembly to the nose cowling. The inner ring may be manufactured with machined strut stubs 102 designed to accept struts which may be fitted later by bolting or welding.
In order to assist with control of the engine it is necessary to measure the stagnation pressure in front of the inner root of the strut stubs. This pressure measurement is electronically fed back to the engine control system, and variable geometry within the engine (Variable Inlet Guide Vane (VIGV)) is altered to suit the measured conditions. Pressure is measured via static pressure measurement holes 20 that are located at the root of the strut stubs and communicate with a pressure manifold 8.
The pressure manifold 8 with which the holes communicate is formed as an annular slot, which may be continuous or discontinuous and is sealed by an end plate 30. The manifold 8 can thus link several or all of the static pressure measurement holes 20 into a single pressure vessel. A transducer in the manifold gives a collective reading for the grouped static pressure measurement holes.
The geometry of static pressure measurement holes is critical for their functionality. In order to work the hole must be normal to the gas flow or angled, for example at 30°, away from the flow as illustrated in FIG. 2. Angling the hole away from the flow discourages ingress of water into the manifold which would adversely affect pressure readings. In order to function correctly, the internal diameter of a static pressure measurement hole must be continuous along its entire length and must remain unobstructed. During operation, ice can build up around the mouth of the hole, partially blocking the hole and adversely affecting readings. In order to prevent this, a heater mat 6 may be bonded to the external surfaces of the inner ring (see FIG. 1). However, any local break in the gas flow over the opening of the hole will adversely affect the readings. If a heating mat is sited close enough to the hole to prevent ice build up from blocking the hole, the presence of the mat will inevitably cause local gas flow disturbance sufficient to prevent the hole from functioning correctly. Such a situation is illustrated in FIG. 2.